Building structural element

ABSTRACT

A building structural element ( 1 ) having a pair of beams ( 5 )joined by a plate member ( 13 ) or alternatively a generally U-shaped channel having a pair of opposed side walls ( 33, 34 ) joined by a further portion ( 35 ). In either case an interior space ( 57 ) is defined by either the pair of beams ( 5 ) and plate member ( 13 ) or by the pair of side walls ( 33, 34 ) and the further portion ( 35 ). A cementitious material, such as concrete, occupies a substantial volume of the interior space ( 57 ) and the building structural element ( 1 ) has a post-tensioned pre-stressing force applied thereto.

FIELD OF THE INVENTION

[0001] This invention relates to a building structural element and amethod of making a building structural element. More particularly thisinvention relates to a building structural element and method of makingsuch an element which is used in floor systems of buildings.

[0002] The invention may also be used for many other building uses, suchas a roof deck over road tunnels, railway tracks and the like. It can beused for large multi-bay floor spaces, particularly with high floor tofloor dimension and/or long spans in one or both directions and/or highfloor loadings such as floor decks for retail, recreational or otheruse. It can also be used for bridge decks and for any other uses wherelong spans and/or high loads have to be carried between supports.

[0003] One particular aspect of the invention relates to a floor beamused in the construction of floor systems of a building and ispreferably made from a combination of two components, preferably madefrom steel, and a cementitious material such as concrete.

BACKGROUND OF THE INVENTION

[0004] Beams are generally required for floor systems of buildings thatspan in excess of 8 to 10 metres. This is typically the upper range orlimit that floor slabs can extend without using beams for structuralsupport. Supporting columns are usually located 6 to 9 metres apart ofwhich a common spacing is 8.4 metres. This is a suitable office modulethat accommodates the widths of a column plus three spaces that may beused for vehicles located between adjacent columns and used for anyparking levels below the office floors (or retail floors, institutionalfloors or other floors) of the building. Beams are required to span thelarger dimension of a rectangular floor panel between a grid of fourcolumns or between a pair of external columns on the outside of thebuilding and the core or central part of a multi storey building. Thecore is typically used to house lift wells and other common roomsaccessible by people on each floor. The floor systems spanning betweenthese beams can and do take on many forms which depends on localavailability, economies and the client's, engineer's or builder'spreference or a combination of preferences of any one of these threeentities.

[0005] Floor systems that span about 8 metres in one direction betweenthe supporting beams can vary from formed concrete slabs on conventionalformwork or large table forms, concrete slabs on metal tray forms thatare temporarily propped on supports, secondary steel beams at closecentres (2.1 to 3.0 metres) supporting a relatively thin slab on anunpropped metal tray forms. Also there are other proprietary systemssuch as various stressed plank systems that may be able to span a 7 to 8metre distance unpropped.

[0006] For the supporting beams the upper range of suitability forreinforced concrete shallow band beams is about 10 to 12 metres. Forpre-stressed shallow band beams (usually post-tensioned) the upper rangeof suitability is generally 12 to 14 metres. However for spans in excessof 12 to 14 metres, special attention and detail is required with commonsolutions being presented by either a steel floor beam system or apre-stressed concrete floor beam system.

[0007] In the steel floor beam system there are a series of steel beamsseparated at their centres between 2.1 to 3.0 metres spanning the largerdistance of the rectangular floor plan and are in turn supported by“primary” beams that span the shorter distance between the supportcolumns. A particular problem that most engineers or designers are facedwith in using a floor beam system is to reduce the amount of deflectionof each beam either due to dead loads or live loads or a combination ofboth. Unnecessarily large deflections and vibrations of the floorsystems can affect the amenity of the floor.

[0008] With the steel floor beam system used at present it has thedisadvantage that each beam needs substantial connections at itssupports. The beams and the connections usually require an appliedprotective coating to give them resistance to fire. Although such steelfloor beams can be made to act compositely with the relatively thinfloor slab that they support using shear studs, there is a small ongoingcomponent of dead load deflection due to creep of the concrete and shearstud interface. Deflections of the steel beams due to dead loads mostlyoccur as soon as the loads are applied and can be allowed for by precambering the beam. However floor deflections and vibrations due totransient live loads are still an inherent problem particularly forlarger spans, as the composite steel beam is less stiff than areinforced concrete or pre-stressed concrete beam that would be used forthe same span.

[0009] The other type of floor beam system that is generally used todayis a post-tensioned, pre-stressed concrete floor beam system. Theyinclude concrete beams that have a deep aspect ratio, in other words thedimensions of the concrete beam are such that it is deeper than itswidth and these concrete beams are generally pre-stressed for spans inexcess of 10 metres. The concrete beams are poured in situ with the slabthat they support. Then pre-stressing is applied by post-tensionedtendons that are stressed when the concrete has attained sufficientstrength usually within 3 to 6 days of pouring. The concrete beam andthe adjacent slab panels are usually formed on large table forms thatare crane lifted from floor to floor. Usually two sets of tables areused to maintain a preferred floor cycle of about one week with half thefloor area poured at a time to provide continuity of work for thevarious trades. Very little prefabrication is possible other than thereinforcing cage for the beam that may or may not include thepre-stressing tendons. Any prefabricated cage needs to be well bracedand cradled to be able to be crane lifted into the beam formwork.Generally no difficult connections exist at the supports of the beam asconcrete stitches the beam to the supports. Where a poured concrete beamis supported at a concrete core that has been “jump formed” ahead of themain floors, the connection can be a simple rebate in the face of thewall of the core and reinforcing bars at the top and bottom of the beamare screwed into ferrules anchored into the core wall. Alternatively thebeam can be seated into pockets left in the core wall.

[0010] The pre-stressed concrete floor system is stiffer than the steelfloor beam system required to span the same distance and is thus lesssusceptible to floor vibrations and deflections due to transient loads.However as concrete creeps under sustained load, the incrementaldeflection of the floor system that occurs after the floor is occupiedis not only that due to live load and light weight partitions, but alsoa significant proportion of deflection from the dead load due to thecreep component. Pre-stressing may balance out most of the deflectiondue solely to the dead load. However, as the axial pre-stressing impartsa permanent axial force to the beam, there are losses of the prestressforce from the resulting time-dependent shortening that will lead tofurther incremental deflection of the beam.

SUMMARY OF THE INVENTION

[0011] An object of the present invention is to provide a buildingstructural element that substantially overcomes one or more of the abovedisadvantages. More particularly the present invention provides abuilding structural element having minimal deflections and substantiallymaintaining pre-stressed force axially therewithin and reduces the lossof such prestress force due to axial creep that shortens beams as inprior art systems.

[0012] According to a first aspect of the invention, there is provided abuilding structural element comprising:

[0013] a pair of beams, each beam in said pair having a first flangeportion, a second flange portion and a web portion extending betweensaid first flange portion and said second flange portion;

[0014] a plate member adapted to engage one of said first flange portionor said second flange portion of each beam such that an interior spaceis defined by each beam in said pair of beams and said plate member;

[0015] wherein cementitious material occupies a substantial volume ofsaid interior space to form a non-unitary building structural elementand said building structural element has residual or no deflection underdead load after application of a post-tensioned pre-stressing force.

[0016] The building structural element may further comprise one or moretendons extending along a length of the interior space defined betweeneach beam and said plate member which may be a metal tray form soffit.Each one or more tendons may be pre-stressed to provide an upwardlydirected force to counteract a portion of the dead load. The firstflange portion of each beam may support part of a floor span after theelement is secured at each end. Typically, the element may extendbetween a column and a core of a building. The element may end a shortdistance from the core and a short distance from the column oralternatively a short distance from a column at each end and istemporarily supported on false work at its end and possibly also at midspan.

[0017] Each beam is preferably constructed of a metal, such as steel,and the cementitious material is preferably concrete.

[0018] According to a second aspect of the invention, there is provideda building structural element comprising:

[0019] a pair of beams, each beam in said pair of beams having a topflange portion, a bottom flange portion and a web portion extendingbetween said top flange portion and said bottom flange portion;

[0020] a plate member adapted to engage respective bottom flangeportions of each beam in said pair of beams such that an interior spaceis defined by each beam in said pair of beams and said plate member;

[0021] wherein cementitious material occupies a substantial volume ofsaid interior space to form a non-unitary building structural elementand said building structural element has a residual or no deflectionunder dead load after application of a post-tensioned pre-stressingforce.

[0022] The plate member may be a metal tray form soffit or othersuitable horizontal soffit surface.

[0023] According to a third aspect of the invention, there is provided abuilding structural element comprising:

[0024] a generally U-shaped channel means having a pair of opposed sidewalls and a further portion joining each side wall;

[0025] wherein said pair of opposed side walls and said further portiondefine an interior space; and

[0026] wherein cementitious material occupies a substantial volume ofsaid interior space and said building structural element has residual orno deflection under dead load after application of a post-tensionedpre-stressing force.

[0027] According to a fourth aspect of the invention, there is provideda method of making a building structural element comprising the stepsof:

[0028] constructing a pair of beams, each beam in said pair of beamscomprising a first flange portion, a second flange portion and a webportion extending between said first flange portion and said secondflange portion;

[0029] forming and assembling a plate member such that the plate memberengages one of either said first flange portion or said second flangeportion of each beam so as to create an interior space defined by saidpair of beams and plate member; and

[0030] pouring a cementitious material into said interior space to forma non-unitary building structural element such that on curing of saidcementitious material and application of a post-tensioned pre-stressingforce, said building structural element has substantially no deflectionunder dead load.

[0031] The pouring step may be done separately or as part of pouring theadjacent floor spans which the element supports. The element may haveits ends initially supported on temporary support structures adjacent tothe permanent end supports of the beam with possible additionalsupport(s) along the span.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] A preferred embodiment of the invention will hereinafter bedescribed, by way of example only, with reference to the drawingswherein:

[0033]FIG. 1 is a plan view of part of a building floor system extendingbetween a core wall of a building and an edge of the building;

[0034]FIG. 2 separated into FIGS. 2A and 2B, is a side sectional viewtaken on the line A-A of FIG. 1;

[0035]FIG. 3 is a sectional view of a structural element in useaccording to one embodiment of the invention taken on the line B-B ofFIG. 2;

[0036]FIG. 4 is a sectional view of the structural element in use takenon the line C-C of FIG. 2;

[0037]FIG. 5 is a sectional view of a structural element in useaccording to a further embodiment and similar to FIG. 3;

[0038] FIGS. 5(a) and 5(d) are side views of the structural element inFIG. 5 showing separate conditions of the structural element;

[0039]FIG. 6 is a sectional view of the further embodiment of thestructural element in use similar to FIG. 4;

[0040]FIG. 7 is a side sectional view showing the structural elementapplied to a tunnel cover;

[0041]FIG. 8 is a side view of a prior art pre-cast structural element;and

[0042]FIG. 9 shows a plan view and a side view of structural elementsacross a floor span with secondary supporting beams.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] Shown in FIG. 1 is a plan view of a pair of structural elements 1that each extend from a perimeter column 2 on an outer edge of abuilding and the core wall 3 in the central part of the building. Thespan of each structural element may extend up to 18 metres in length andbeyond and the separation between each element 1 is dependent on theseparation between each perimeter column which as previously discussedmay be equivalent to fit three spaces for cars in parking levelsunderneath the office floors which typically may be anywhere between 6to 9 metres. Between the perimeter columns 2 extends an edge beam 16 andthe floor 4 extends between the adjacent elements 1. The edge beam 16can incorporate an inner steel beam that stops just short of the sidesof the perimeter columns 2, so as not to require any physical connectionto the column 2 and is supported on the same external support frame 19that supports the main structural element 1. This steel component of theedge beam 16 is precambered to take the dead loads, allowing minimaldeflections along the edge of the building that may affect any facadeglazing. It is to be noted that in FIG. 1 although only two beams areshown any number of beams needed to support a designated floor area maybe used.

[0044] With reference to FIGS. 3 and 4 the element 1 is essentiallyconstructed of an outer shell, typically made from steel which comprisesfirst and second (side shell) beams 5 generally in the shape of I-beamseach having a web portion 30 and at either end of the web portion 30exists first and second flange portions 31 and 32. Extending at thelower portion of the element 1 is a plate member or (form tray decksoffit) 13 generally made from metal and more particularly steel wherebythis extends between each bottom or second flange portion 32 of the sideshell beams 5 and is affixed or otherwise engaged with the flangeportions 32. Shear studs 12 extending from the web portion 30 of theside shell beams 5 are used to obtain integral action with thecementitious material that is poured into an interior space 57 of theshell defined by each beam 5 and the plate member 13. Ligatures 23generally in the form of a U-shape extend from within the floor 4downwardly into the interior space 57 and through raised ribs of theplate member 13. This is also used for extra support for thecementitious material that is poured into the interior of the shell.Furthermore reinforcement bars 24 are also provided within the element1. A closer or spacer 15 also is positioned within the floor 4 to tiethe cage structure to the side shell beams 5. Pre-stressed tendonelements 10 are several in number and shall be described with referenceto FIG. 2 hereinafter.

[0045] It is to be noted that the pair of beams 5, and variousreinforcing for the beam can be pre-fabricated off site in a controlledproduction line environment and be transported to the site with itsreinforcement bars 24 and ligatures 23, pre-stressed tendon elements 6,8 and 10 and bracing 15 to keep the reinforcing elements in place.Tendon element 6 is a dead end anchor for one of the tendons 10 andtendon element 8 is a pressed metal form incorporating a live end anchorrecess. The two steel side shell beams 5, coupled with bracing 25 forhandling and transport, enable the complete assembly to be delivered toa site and in one simple lift, generally of around 4.5 tonnes for an 18metre beam, be in place on pre-set support frames at its two ends, thesupport frames being designated by 18 and 19 and at its mid span withsupport frame 20.

[0046] With reference to FIG. 2 at the core wall 3 of the building theelement 1 may be housed adjacent to a rebate 22 in the wall 3 and theindividual reinforcement bars 14 and 17 screwed into ferrules 21 thathave been precast into the jump form core wall 3 for both the top andbottom reinforcing connections to respectively go to reinforcement bars14 and 17. At the other end of the beam, connection to the externalcolumn 2 all that is required is that the connecting reinforcing bars 14and 17 extend the required length past the face of the column 2. Thesereinforcing bars 14 and 17 together with the beam ligatures 11 at theend sections of the element 1 are the only reinforcing steel for theelement 1 that needs to be fixed on site.

[0047] Whilst small (up to 150 mm diameter) penetrations for firesprinkler pipes, sewer pipes etc can be accommodated through the web ofthe element 1, each end of each beam can be stepped up to accommodateany major service duct reticulation without impinging on the ceilingheight by simply stepping up the bottom flange portion 32 of each steelside shell beam 5. This is more clearly shown in FIG. 2 where the step 7is shown such that the space between the step and either the core wall 3or column 2 designated as 9 provides a space for such a service ductreticulation if required. This step also allows the housing of the liveor jacking end for one or more pre-stressed tendons 10. The tendons 10as mentioned are post-tensioned on site and may include any number asdesired to be located in the interior space between each steel sideshell beam 5. Shown in FIG. 2 a pre-stressed tendon extends between thesteps 7 in the bottom flange portion 32 and further pre-stressingtendons 10 are shown extending the full length of the beams and having adrape, in other words stressed in a concave manner and this is done toprovide an uplift force or component that cancels out anywhere between50% to 100% of the total dead load, although about 50-60% is usuallysufficient for these beams, with the balance of the dead load andtransient load capacity being provided by the steel shells of theprefabricated beam and the cementitious material. The dead load isdeemed that weight comprised of the floor itself, the beams andpermanent superimposed loads such as ceiling surfaces and floorfinishes. The two steel side shell beams 5 are generally precambered sothat once the temporary props are removed all of the deflection due tothe dead load has been accommodated. In reality, the steel side shellbeams 5, the pre-stressing tendons and the concrete core filling theinterior space of the pair of beams 5 interact together to share thetotal load. Nevertheless, the beams 5 and the pre-stressing tendonssubstantially prevent the reinforced component from taking anysignificant proportion of the load and certainly will not allow theconcrete component to creep substantially as the pre-stressing tendonsplus the precambered steel shell beams 5 between them are capable oftaking the total dead load before the beam has deflected back to thehorizontal.

[0048] In use, as mentioned, before the construction of the side shellbeams 5, shear studs and prestress tendons 10 and most of the beamsreinforcement are done offsite and the reinforcing members 14, 17 andligatures 11 to the end section are done on site. The element 1 is thenplaced on its temporary supports just short of the rebate 22 in the corewall and the column at the other end. The reinforcing bars 14 and 17 arethen placed into the respective ferrules and together with the ligatures11 placed on site and thereafter the floor is poured such that thecementitious material, or generally concrete, is poured into the floorstructure and allowed to fill a substantial volume of the interior space57 within the side shell beams 5. The combination of the two steel sideshell beams 5 and the cementitious (concrete) material act integrallyfirstly by the concrete being poured between and over the steel beamsand also via the shear studs 12 which are shop welded to the insides ofeach of the webs 30 of the beams 5.

[0049] Thus the two side steel beams 5 significantly increase the axialsteel content of the composite beam which acts as a compression memberunder the action of the pre-stressing load, such that loss of anyprestress force due to axial creep shortening of the beam is minimal.This hybrid beam construction also then can have no overall deflectionfrom dead load or it can be provided with a slight residual upwardscamber before the transient loads. The remaining robust concrete beam asan uncracked section is available to assist the other two componentswith the incremental deflections due to the live load and any othertransient loads.

[0050] As the external columns do not initially support any of the floor4 and element 1, the floor 4 and element 1 and columns 2 could be pouredthe same day. The columns 2 may be poured whilst the floor formwork andreinforcing to the floor panels 4 between the hybrid elements 1 is beinginstalled. It is envisaged that with a well organised work force, evenfor large floor areas, three day floor cycles or even less couldregularly be achieved by pouring half the floor on the first day,pouring the other half on the second day and preparing columns and liftperimeter shutters etc on the third day.

[0051] The concrete part of the element 1 is proportioned to take thereduced fire design load condition being 1.1 times the dead load plus0.4 times the live load in Australia and similar figures used in othercountries. Thus there is no need for either fire protection or acomplicated fire engineering analysis if this can in fact provide asolution. The combined capacity of the steel and pre-stressed concretebeams is more than adequate to carry the in-service load which isgenerally 1.25 times the dead load plus 1.5 the live load in Australiaand similar figures are used in other countries.

[0052] Most codes set deflection limits for incremental deflection, dueto any creep component of the dead load plus the deflection due to liveload and plus the weight of the partitions, at span/500. Thus for an 18metre span this limit is 36 mm. For any building with large internalspans parallel to and adjacent to the side of the building, thismagnitude of differential deflection between the floor next to a columnat the side of the building, where there is zero deflection, and the midspan deflection at the adjacent internal beam, which may be as close as2.5 metres for a steel beam floor system, is obviously too much. Morestringent deflection criteria than the code requires may be necessary.The present invention through its hybrid steel and concrete beam canachieve maximum incremental deflections of the order of under 20 mm forsuch 18 meter spans.

[0053] With reference to FIGS. 5 and 6 there is shown a furtherembodiment of the present invention wherein the structural element 1 ismade from a single unitary or joined construction. Specifically, insteadof having a pair of opposed beams linked by a plate member as in thefirst embodiment, the embodiment in these Figures has a generallyU-shape channel formed of a pair of side walls 33 and 34 and a bottom orfurther portion 35 linking each of the side walls 33 and 34. Preferably,the construction is made out of steel. Upper portions of the side walls33 and 34 respectively have flange elements 36 and 37 for supportingpart of the floor 4.

[0054] With reference to FIGS. 5(a), (b), (c) and (d) the structuralelement can be made from either a single piece of steel plate forexample utilising four folds indicated in FIG. 5(a) at 38, 39, 40 and 41(with no welding). Alternatively, the construction could be welded atpoint 42 and retain the four folds 38 through to 41 as shown in FIG.5(b) giving a two plate construction. In FIG. 5(c) there is shown analternative arrangement for the structural element retaining folds 38and 41 but welded at points 43 and 44, providing a three plateconstruction. Finally in FIG. 5(d) five pieces of steel plate could beused with no folds and four welds as indicated at points 45, 46, 47 and48.

[0055] The invention can also be used for shorter spans using shallowerside shell steel beams with or without the use of pre-stressing tendonsand with or without notches in the bottom of the ends of the beam toaccommodate major service duct reticulation.

[0056] Where pre-stressing tendons are used for such shallow beams thatdo not have service duct step-ups that double as stressing anchor pointsthen the anchors can be stressed from rebate pockets in the top of thefloor that are filled in later after stressing and grouting of thetendons.

[0057] With reference to FIG. 7, and as mentioned previously, theinvention is suitable for use in road tunnels using a “cut and cover”method. This involves lifting into position each of the structuralelements, which already have their reinforcement and prefabricated inplace, using relatively low load capacity cranes. This compares toextremely heavy precast pre-stressed concrete beams 49 used in prior artsystems and shown in FIG. 8. The heavy precast pre-stressed concretebeams 49 have required extremely large cranes to be used for suchlifting applications. With reference to FIG. 8 the beams 49 haverelatively thin flanges 50 and a topping slab 51 normally used for suchapplications. There is usually the necessity for tanking or using awaterproof membrane 52 over the full extent of the deck, which in turnrequires a protective wearing slab 53. All of these can be dispensedwith by using a watertight integral pour that is possible with the useof the present invention. With reference back to FIG. 7 this involvesthe use of fluid barrier means or more particularly water stops 58 andlocal tanking (or using waterproof membranes) 54 fitted to ensure thatthe roof of the tunnel is watertight. The floor deck 55 of the spanbetween the structural elements 1 can be sufficiently thick and pouredin sections of suitable size and, if necessary, pre-stressed betweencontrol joints 56. The water stops 53 sand the local tanking ormembranes 54 are fitted to each of the control joints 56.

[0058] With reference to FIG. 9 and as mentioned previously, theinvention is also suitable for large multi-bay floor spaces,particularly with high floor to floor dimension and/or long spans in oneor both directions and/or high floor loadings such as floor decks forretail, recreational or other use. The building element (1) can be usedto span between columns (59), to which secondary beams (60) are bolted(61), supporting the floor slab (4) between the structural elements (1).

[0059] The structural elements can be tailored to span the self-weightof the floor structure, plus construction live load during constructionas simply supported between columns.

[0060] The structural element then forms a reinforcement concrete orpre-stressed concrete element that is continuous over several spans forthe loads that need to be supported by the floor deck.

1. A building structural element comprising: a pair of beams, each beamin said pair having a first flange portion, a second flange portion anda web portion extending between said first flange portion and saidsecond flange portion; a plate member adapted to engage one of saidfirst flange portion or said second flange portion of each beam suchthat an interior space is defined by each beam in said pair of beams andsaid plate member; and wherein cementitious material occupies asubstantial volume of said interior space to form a non-unitary buildingstructural element and said building structural element has residual orno deflection under dead load after application of a post-tensionedpre-stressing force.
 2. An element according to claim 1 furthercomprising one or more tendons extending along a length of said interiorspace.
 3. An element according to claim 2 wherein said one or moretendons may each be pre-stressed to provide an upwardly directed forceto counteract a portion of said dead load.
 4. An element according toany one of the previous claims that is used in supporting long-spanfloor areas within a building.
 5. An element according to claim 4wherein said first flange portion in each beam supports part of a floorspan within said building.
 6. An element according to claim 5 whereinone end of said element is secured to a column of said building and theother end of said element is secured to a core of said building.
 7. Anelement according to claim 6 wherein prior to securing each end of saidelement, said element is temporarily supported on false work at saideach end of said element.
 8. An element according to any one of theprevious claims having one or more shear studs extending into saidinterior space from either said web portion in each said beam so as toobtain integral action with said cementitious material.
 9. An elementaccording to any one of the previous claims wherein support means, inthe form of ligatures, extend into said interior space to support saidcementitious material.
 10. An element according to any one of theprevious claims further comprising a plurality of reinforcement barsextending along a portion of said element.
 11. An element according toclaim 10 wherein said other end of said element is housed adjacent arebate in said core of said building.
 12. An element according to claim11 wherein individual ferrule means formed in said core are secured toindividual reinforcement bars of said plurality of reinforcement bars.13. An element according to any one of the previous claims wherein eachend of said each beam is stepped to accommodate service ducts.
 14. Anelement according to any one of the previous claims wherein each beam insaid pair of beams is constructed of metal.
 15. An element according toany one of the previous claims wherein said cementitious material isconcrete.
 16. A building structural element comprising: a generallyU-shaped channel means having a pair of opposed side walls and a furtherportion joining each side wall; wherein said pair of opposed side wallsand said further portion define an interior space; and whereincementitious material occupies a substantial volume of said interiorspace to form within the channel means a non-unitary building structuralelement and said building structural element has residual or nodeflection under dead load after application of a post-tensionedpre-stressing force.
 17. An element according to claim 16 having one ormore shear studs extending into said interior space from either of saidside walls so as to obtain an integral action with said cementitiousmaterial.
 18. An element according to either claims 16 to 17 whereinsupport beams, in the form of ligatures, extend into said interior spaceto support said cementitious material.
 19. An element according to anyone of claims 16 to 18 further comprising one or more tendons extendingalong a length of said interior space.
 20. An element according to claim19 wherein said one or more tendons may each be pre-stressed to providean upwardly directed force to counteract a portion of said dead load.21. An element according to any one of claims 16 to 20 wherein each ofsaid side walls has a flange element extending from a free end of eachside wall, said flange element supporting part of a floor deck.
 22. Anelement according to any one of claims 16 to 21 wherein each end of saidelement is secured to a support structure.
 23. An element according toclaim 22 wherein prior to securing each end of said element to saidsupport structure, said element is temporarily supported on false workat said each end of said element.
 24. An element according to any one ofclaims 16 to 23 further comprising a plurality of reinforcement barsextending along a portion of said element.
 25. An element according toclaim 24 wherein at least one of the ends of said element is housedadjacent a rebate in said support structure.
 26. An element according toclaim 25 wherein individual ferrule means formed in said supportstructure are secured to individual reinforcement bars of said pluralityof reinforcement bars.
 27. An element according to any one of claims 16to 26 wherein each end of said each beam is stepped to accommodateservice ducts.
 28. An element according to any one of claims 16 to 27wherein said channel is unitary in construction.
 29. An elementaccording to any one of claim 16 to 27 wherein said channel means isconstituted by one or more joints joining adjacent portions.
 30. Anelement according to any one of claims 16 to 29 wherein said channelmeans is constructed of metal.
 31. An element according to any one ofclaims 16 to 30 wherein said cementitious material is concrete.
 32. Anelement according to any one claims 16 to 31 wherein said floor deck hasfluid barrier means to ensure said element is fluid tight.
 33. Anelement according to claim 32 wherein said fluid barrier means includesa fluid stop and a fluid proof membrane.
 34. A building structuralelement comprising: a pair of beams, each beam in said pair of beamshaving a top flange portion, a bottom flange portion and a web portionextending between said top flange portion and said bottom flangeportion; a plate member adapted to engage respective bottom flangeportions of each beam in said pair of beams such that an interior spaceis defined by each beam in said pair of beams and said plate member; andwherein cementitious material occupies a substantial volume of saidinterior space to form a non-unitary building structural element andsaid building structural element has a residual or no deflection underdead load after application of a post-tensioned pre-stressing force. 35.A structural element according to claim 34 wherein said plate member isa soffit.
 36. A structural element according to claim 35 wherein saidsoffit is a metal tray form soffit.
 37. A method of making a buildingstructural element comprising the steps of: constructing a pair ofbeams, each beam in said pair of beams comprising a first flangeportion, a second flange portion and a web portion extending betweensaid first flange portion and said second flange portion; forming andassembling a plate member such that the plate member engages one ofeither said first flange portion or said second flange portion of eachbeam so as to create an interior space defined by said pair of beams andsaid plate member; and pouring a cementitious material into saidinterior space to form a non-unitary building structural element suchthat on curing of said cementitious material and application of apost-tensioned pre-stressing force, said building structural element hassubstantially no deflection under dead load.
 38. A method according toclaim 37 wherein said pouring step involves pouring said cementitiousmaterial as part of a floor deck and is poured into said interior spacein the same pouring step as an adjacent floor deck.
 39. A methodaccording to claim 37 wherein said pouring step involves pouring saidcementitious material in constructing a floor deck and said pouring isdone separately to the pouring step in constructing said floor deck. 40.A method according to any one of claims 37 to 39 further comprisingsupporting the ends of said structural element initially on temporarysupport structures adjacent to a permanent end support of saidstructural element.